(a) Field of the Invention
The present invention relates to data transmission systems and more particularly to a system for recovering the clock for a data transmission receiver.
(b) Description of the Prior Art
A typical data transmission system consists of a transmitter which transmits data, in the form of binary pulse trains, to a receiver over a serial transmission link. Both the transmitter and receiver are provided with a local clock and typically the durations of the pulses transmitted are much greater than the period of the local clock. The local clocks are required to have exactly the same frequency and are controlled, for example, by a crystal oscillator.
The clock recovery circuit according to the invention allows the receiver to acquire the signal present on the data transmission link at the correct instant of time. The clock signal can be transmitted from the transmitter in either of two ways:
(a) over a transmission channel materially different from that used for the data signal; or PA1 (b) over the same channel as the data signal.
In the first case, the transmission link will be limited in distance and in frequency because of the phase difference that exists in two signals which are transmitted over different channels. In the second case, which corresponds to the present invention, there already exist systems performing this same function. However, the system described herein has the advantage of synchronizing from the very first data signal pulse, thereby eliminating the presynchronization period normally required in prior art systems.